This invention relates to a transmission which includes a main transmission mechanism equipped with gear trains of a plurality of transmission stages disposed between an input shaft capable of being connected to a crank shaft of an engine and an output shaft parallel to the input shaft and a sub-transmission mechanism transmitting the driving force from the input shaft to a driven gear of the output shaft via gear trains of other transmission stages, and which is supported while projecting from an engine case along the axis of the crank shaft.